The present invention relates to a semiconductor laser having a layer sequence forming a heterostructure diode wherein the laser active zone is interposed between semiconductor layers whose band gaps are greater than the greatest band gap within the laser active zone.
Various requirements must be met by semiconductor lasers if they are to be employed in optical data transmission systems. For example, a semiconductor laser used for this purpose should have a linear light-current characteristic, stable emission without fluctuations and be easy to manufacture.
U.S. application Ser. No. 41,171 filed May 21, 1979 by Marschall et al. now U.S. Pat. No. 4,278,949, discloses an easily manufactured laser which meets the above requirements. In accordance with the teachings of this application, the current flowing in the forward direction of the laser diode is constricted to a narrow, strip-shaped region of the laser active zone by providing a trough-like recess extending perpendicularly to its radiation emitting surface. Through this recess, a narrow semiconductor region of essentially uniform conductivity type and extending to the laser active zone is produced by diffusion of doping material. However, this laser exhibits a multimode spectral emission which constitutes a disadvantage for some applications such as the broadband optical transmission of data.
German Offenlegungsschrift No. 27 10 813 discloses that a single mode spectral emission can be realized in a laser having low threshold current densities by providing external wave guidance which is not only perpendicular but also parallel to the active zone. For this purpose, a strip-shaped current path is produced by diffusion so as to penetrate into the active zone thereby increasing the index of refraction. Although such a laser is easy to manufacture, the diffusion zone extends into the recombination region where it can seriously inhibit laser emission thereby leading to rapid aging of the laser. Also, the single mode spectral emission obtained with this laser, which depends upon the temperature of the laser, is not entirely satisfactory.
It is therefore an object of the present invention to overcome these disadvantages. In particular, a laser is provided which has an improved single mode spectral emission even under conditions in which the temperature of the laser fluctuates. Further, the laser can be manufactured by simple techniques and has a long service life.